It's entirely possible that LH have two providers, perhaps Deutsche Telekom AG (DTAG) and another. The requirement to have diversity and separacy may have been delivered on day 1 however if the second provider had used DTAG and pointed out the requirement, over time this can become compromised. Airports are prone to lots of construction so existing telco cables can be moved to avoid civils works. If system "A" is at risk due to a new roundabout and is migrated by DTAG to the ducts carrying system "B", all the eggs are in one basket. Along comes another civils work where the digger operator is gung ho and the holes in the swiss cheese all line up.
Teasing out who is to blame is far from easy.